Language Integrated Query(LINQ)
What is LINQ?
Language Integrated Query (LINQ, pronounced x"link") is a Microsoft .NET Framework component that adds native data querying capabilities to .NET languages using a syntax reminiscent of SQL. Many of the concepts that LINQ has introduced were originally tested in Microsoft's Cω research project. LINQ was released as a part of .NET Framework 3.5 on November 19, 2007.
Standard Query Operators
The set of query operators defined by LINQ are exposed to the user as the Standard Query Operator API. The query operators supported by the API are
Select / SelectMany
The Select statement is used to perform a projection on the collection to select either all the data members that make up the object or a subset of it. The SelectMany operator is used to perform a one-to-many projection, i.e., if the objects in the collection contain another collection as a data member, SelectMany can be used to select the entire sub-collection. The user supplies a function, as a delegate, which selects the data members. The delegate is invoked on all the objects to project out the unneeded data members. Selection creates an object of a different type, which has either same or as many data members as the original class. The class must be already defined for the code to be compilable.
Where
The Where operator allows the definition of a set of predicate rules which are evaluated for each object in the collection, and objects which do not match the rule are filtered away. The predicate is supplied to the operator as a delegate.
Join / GroupJoin
The Join operator performs an inner join on two collections, based on matching keys for objects in each collection. It takes two functions as delegates, one for each collection, that it executes on each object in the collection to extract the key from the object. It also takes another delegate via which the user specifies which data elements, from the two matched elements, should be used to create the resultant object. The GroupJoin operator is used to perform a group join. Like the Select operator, the results of a join are instantiations of a different class, with all the data members of both the types of the source objects, or a subset of them.
Take / TakeWhile
The Take operator is used to select the first n objects from a collection, while the TakeWhile operator, which takes a predicate, selects those objects which match the predicate.
Skip / SkipWhile
The Skip and SkipWhile operators are complements of Take and TakeWhile - they skip the first n objects from a collection, or those objects which match a predicate (for the case of SkipWhile).
OfType
The OfType operator is used to select the elements of a certain type.
Concat
The Concat operator concatenates two collections.
OrderBy / ThenBy
The OrderBy operator is used to specify the primary sort ordering of the elements in a collection according to some key. The default ordering is in ascending order, to reverse the order the OrderByDescending operator is to be used. ThenBy and ThenByDescending specifies subsequent ordering of the elements. The function to extract the key value from the object is specified by the user as a delegate.
Reverse
The Reverse operator reverses a collection.
GroupBy
The GroupBy operator takes a delegate that extracts a key value and returns a collection of IGrouping objects, for each distinct key value. The IGrouping objects can then be used to enumerate all the objects for a particular key value.
Distinct
The Distinct operator removes duplicate instances of a key value from a collection. The function to retrieve the key value is to be supplied as a delegate.
Union / Intersect / Except
These operators are used to perform a union, intersection and difference operation on two sequences, respectively.
EqualAll
The EqualAll operator checks if all elements in two collections are equal.
First / FirstOrDefault / Last / LastOrDefault
These operators take a predicate. The First operator returns the first element for which the predicate yields true or throws an exception if nothing matches. The FirstOrDefault operator is like the First operator except that it returns the default value for the element type (usually a null reference) in case nothing matches the predicate. The last operator retrieves the last element to match the predicate, or throws an exception in case nothing matches. The LastOrDefault returns the default element value if nothing matches.
Single
The Single operator takes a predicate and returns the element which matches the predicate. An exception is thrown if none or more than one elements match the predicate.
ElementAt
The ElementAt operator retrieves the element at a given index in the collection.
Any / All / Contains
The Any operator checks if there are any element in the collection matching the predicate. It does not select the element, but returns true for a match. The All operator checks if all elements match the predicate. The Contains operator checks if the collection contains a given value.
Count
The Count operator counts the number of elements in the given collection.
Sum / Min / Max / Average / Aggregate
These operators take a predicate that retrieves a certain numeric value from each element in the collection and uses it to find the sum, minimum, maximum, average or aggregate values of all the elements in the collection, respectively.
Hope these are useful.
